This invention relates to a method for manufacturing a stranded conductor constituting of insulated strands in which an oxide of metal, of which the strands are made, is formed on the surfaces of all the electroconductive strands in an electroconductive conductor and more particularly to a method for forming an electrically insulating layer in a stranded state on the surface of all the strands of the electroconductive conductor.
An increase in power transmission voltage and an increase of the current carrying capacity become prominent so as to cope with a recent increase in power transmission capacity in a power transmission system, and a conductor size of 3,000 to 6,000 mm.sup.2 has been put into practical use. If in this way the conductor size is increased, an alternating current loss is markedly increased dependent upon the influence of the skin effect and proximity effect. To slove this problem, it is known to provide segmental conductors (normally four to eight) into which a stranded conductor is divided with the respective segment electrically insulated from each other. It is also known to provide a stranded conductor made of insulated strands having an electrically insulating film. It is known that the stranded conductor made of all electrically insulated strands is smaller in its AC effective resistance than the above-mentioned segmental conductor.
A method for the manufacture of a conductor made of electrically insulated strands includes a method for providing a strand by forming an electrically insulating layer around the respective strands and twisting the strands and a method for twisting bare strands to provide a conductor and for forming an electrically insulating layer on the respective bare strands.
Where the electrically insulating layer is an enamel coating, the manufacturing method is restricted to the former method, but if the electrically insulating layer is an oxide film of metal, of which the strand is made, such as copper or aluminium, either method may be used.
In the method of forming a conductor with an electrically insulating layer formed on the strands, a plurality of strands pass in parallel and an electrically insulating layer is formed around the strands, whereby the manufacturing efficiency is enhanced. If the conductor size is 3,000 to 6,000 mm.sup.2, the numbers of strands required will be about 500 to 1,000. If the formation of the electrically insulating layer is achieved for a short time, it is necessary to increase the number of equipment. Where the number of equipment is smaller, a longer time is required. In order to make the outer diameter of the conductor of such a larger size as small as possible, a compact-stranded conductor is sometimes used. In such conductor, the strands are twisted with an insulating layer formed thereon to provide a conductor. If such conductor is compacted, the insulating layer of the respective strand is damaged and the insulating effect of the conductors is decreased. Thus, this method cannot be considered to be adopted.
In a method for forming a stranded conductor and then forming an electrically insulating layer on all the strands, the respective strands are exposed in an oxidizing atmosphere (gas or liquid) with the stranded state. In this case, it is easy to subject the outer surface portion of the strand to oxidation treatment. However, difficulty is encountered in effecting adequate oxidation treatment up to the central portion of the conductor. In order to solve such difficulty, a method is known which imparts an undulating bending at the oxidation treatment section to the passing strand or imparts slight untwisting to the stranded conductor to permit interstices to be left between the adjacent strands in the stranded conductor. In this method, an oxidation treatment solution is prevented from penetrating up to the central portion of the stranded conductor by air present or left among interstices of strands at the central portion of the stranded conductor or by a degreasing treatment solution penetrating at a degreasing treatment step before the oxidation treatment step. This method, however, does not assure a positive formation of an adequate oxidation film on the inner strands in the stranded conductor.
An object of this invention is to positively form an oxidation film having an adequate electrical insulation on the outer portion of a stranded conductor or the surface of the strands present in the central portion of the stranded conductor.
Another object of this invention is to permit an oxidation treatment solution to be readily penetrated without preventing the oxidation treatment solution from being penetrated during the oxidation treatment step into the interstices of the strands in the inner portion of the stranded conductor by the presence of air left in the inner portion of the stranded conductor before the stranded conductor is done with the oxidation treatment step, or by the presence of a degreasing treatment solution left penetrated in the inner portion of the stranded conductor in the degreasing step before the oxidation treatment step.
Another object of this invention is to make the oxidation reaction of the surface of the strand more effective by not only flowing an oxidation treatment solution in the radial direction of the stranded conductor, but also flowing it along the interstices of the stranded conductors.